The present invention relates to modulating electromagnetic or acoustic energy, and, more specifically, to modulating such energy so that when the modulated energy is transmitted and a portion thereof is reflected from an object and received at a processing station, the received energy may be processed by a simple signal processor, without regard for relative radial velocity between the object and a sensor or detector of the received energy.
When transmitting energy, such as for example in electromagnetic form in air and acoustic, e.g., sub-sonic, sonic or supersonic, form in water, as may be exemplified by radar and active sonar systems, respectively, it may be necessary or desirable to modulate the transmitted energy, or carrier wave, with a modulating wave having a predetermined frequency characteristic. Such modulation may be used, for example, in cases where it is desired to inject a predetermined amount of energy into a transmitting medium during a predetermined interval or pulse. The transmitting medium may not be able to support a continuous stream of energy having the same frequency characteristics over the entire interval without smearing, or otherwise undesirably distorting, the transmitted wave. Such smearing or distortion will adversely affect or degrade overall operation of a detector system, such as radar or sonar which typically includes an antenna or hydrophone, respectively, by reducing resolution and/ or decreasing the signal-to-noise ratio of the system because the smearing or distortion will also be present in any received wave.
In order to combat such degradation, while still maintaining the capability for injecting relatively large amounts of energy into the transmitting medium during relatively long intervals, it is known to be beneficial to divide the total desired transmitted energy pulse into a plurality of contiguous time intervals, with each interval modulated by a different modulating signal or waveform. It is also known that it is beneficial to frequency modulate (FM) a relatively long pulse and correlate a received or reflected pulse with the modulated transmitted pulse for improving detection and range resolution of a system.
Prior approaches for combining hyperbolic frequency modulated waveforms for accurately determining range to an object or target have utilized hyperbolic waveform sections which had matching or equal time duration and varying start and end frequencies. These techniques require rather complex circuitry, especially with respect to delay circuitry, for combining matched filter outputs, or correlation results, from moving targets.
A prior method for modulating a carrier wave is illustrated in FIG. 1A. The modulating waveform is upwardly swept, or increases in frequency, between starting and ending frequencies lying on curves A and B, respectively, during each portion, or sub-interval, T.sub.1, T.sub.2, T.sub.3, etc. of the overall energy pulse E, as shown in FIG. 1B, for producing a wavetrain including waveforms M.sub.1, M.sub.2 and M.sub.3. The modulating wavetrain may be repeated for a next energy pulse.
The starting frequency f.sub.1 of the first segment M.sub.1 of the modulating waveform is selected to be within the bandwidth of a receiver and the modulating waveform is swept to frequency f.sub.2 which is also within the bandwidth of the receiver. At the end of first sub-interval T.sub.1, a next modulating waveform is selected to begin at a frequency that is a multiple (1+a) (wherein a is greater than zero) times frequency f.sub.1 and end at a frequency that is the same multiple (1+a) times frequency f.sub.2. Likewise, at the end of second sub-interval T.sub.2, another modulating waveform is selected to begin at a frequency that is a multiple (1+a) times the starting frequency of the previous modulating waveform and end at a frequency that is the same multiple (1+a) times the ending frequency of the previous modulating waveform. Thus, the starting and ending frequencies of the modulating waveforms as illustrated in FIG. 1A form a geometric progression with the ratio between corresponding successive elements being multiple (1+a).
Frequency values f.sub.1, (1+a)f.sub.1, (1+a).sup.2 f.sub.1 lie on curve A which is a segment of an exponential. Likewise frequency values f.sub.2, (1+a)f.sub.2 and (1+a).sup.2 f.sub.2 lie on curve B which is a segment of another exponential. Further, the actual modulating frequencies for the corresponding sub-intervals of energy between each of curves A and B along curves M.sub.1, M.sub.2 and M.sub.3 are selected to lie on curves M.sub.1, M.sub.2 and M.sub.3 which are hyperbolic.
A radial component of relative velocity between a receiver and an object manifests itself as a time compression or frequency shift, generally referred to as a Doppler frequency shift or more simply a Doppler shift, of a transmitted wave that is reflected from the object and detected or sensed at the receiver. By radial component of relative velocity is meant the component of relative velocity that exists between one object (such as a detector or receiver) and another object (such as a target of interest) along a straight line between the one and another object. For example, if a transmitted wave is modulated in accordance with the modulating pattern or wavetrain shown in FIG. 1A, and there exists a radial component of relative velocity between a detector and an object impinged by the transmitted wave such that there is a closing or decreasing range, then the frequency modulation from reflections of the transmitted wave by the object will be detected at an apparent frequency higher than that actually transmitted. The detected shift in frequency from that actually transmitted will be a percentage of the transmitted frequency proportional to the magnitude of the radial component of relative velocity between the object and the receiver, so that higher modulating frequencies will be shifted more than lower modulating frequencies. This apparent shift in frequency is known as a Doppler shift, with the amount of frequency shift referred to as the Doppler frequency.
In order to determine the actual position or range to the reflecting object, correlation techniques are frequently employed. For basic correlation, a replica of a transmitted wave is stored at the transmitter and is compared in frequency to a received wave to obtain a maximum overlap, or correspondence, between the two. The time at which the maximum correspondence occurs with respect to the time of the initial transmitted wave is indicative of the range to the target. With knowledge of the speed of propagation of the transmitted and reflected wave in the transmitting medium, the actual range to the object can be determined.
For the modulating waveforms M.sub.1, M.sub.2 and M.sub.3 the Doppler frequency for corresponding points m.sub.1, m.sub.2 m.sub.3 along modulating curves M.sub.1, M.sub.2 and M.sub.3 will be different from each other when received from an object, and a customized correlation system will be required for each of modulating curves M.sub.1, M.sub.2 and M.sub.3. For example, if points m.sub.1, m.sub.2 and m.sub.3 each occur at a same predetermined time from the commencement of their respective modulating curve M.sub.1, M.sub.2 and M.sub.3, then the Doppler frequency detected in response to point m.sub.3 will be greater than that for point m.sub.2, which in turn will be greater than that for point m.sub.1, assuming that the transmitted wave impinges the same object having a closing component of radial velocity. Because of the differences in Doppler frequency among points m.sub.1, m.sub.2 and m.sub.3 for the same target and among all other corresponding points of waves M.sub.1 , M.sub.2 and M.sub.3, the respective relative times from initial energy transmission at which correlation for waves M.sub.1, M.sub.2 and M.sub.3 occurs will differ. If the radial distance between the object and the receiver is decreasing, the correlation time for M.sub.3 will be the shortest, for M.sub.2 the next shortest and for M.sub.1 the longest. Thus it is not possible to ascertain at the receiver simply by noting the respective correlation times which correlation time corresponds to the true range to the object. Generally some form of hypothesis or estimation based on expected Doppler shifts is used in conjunction with the correlation times for determining the actual range.
Referring to FIG. 2, a block diagram of a known correlation system is shown. The correlation system includes a plurality of correlation means 10, three of which are shown, each correlation means 10 including matched filter circuitry 12 and envelope detector circuitry 14, having an input connected to an output of matched filter circuitry 12, a plurality of delay means 30, 32 and 34 having an input connected to an output of corresponding envelope detector circuitry 14, and a summer 40, having a plurality of inputs connected to outputs of corresponding envelop detector circuitry 14. Matched filter circuitry 12 may all be the same, such as for example a delay line correlator or a convolution processor using fast Fourier transforms. Each of matched filter circuitry 12 is conditioned or tuned to respond efficiently to the modulating waveform of a corresponding subinterval of the modulated pulse of energy. Each of matched filter circuitry 12 has an input connected to be supplied with received energy from a detector (not shown), such as an antenna or hydrophone, wherein the received energy may be a portion of the modulated transmitted energy that has been reflected from an object of interest. Another input of each of matched filter circuitry 12 is for receiving a replica of transmitted energy for comparison with the received energy.
Envelope detectors 14 may all be the same, such as for example a full wave rectifier. Each of envelope detectors 14 is conditioned or tuned to respond efficiently to the output signal from a corresponding matched filter circuitry 12.
The output of each of envelope detectors 14, having a correlation signal available thereat, is respectively connected to the input of a corresponding plurality of delay means 30, 32 and 34. Each of delay means 30, 32 and 34 may include a plurality of delay circuitry, such as delay lines, for delaying the correlation signal received at the input a respective predetermined interval of time. Delay circuitry 30a through 30m of delay means 30 corresponds to a Doppler hypothesis, or an expected Doppler shift, due to anticipated relative velocity between the target of interest and the receiver. Each of delay circuitry 32a through 32m, and 34a through 34m corresponds to the respective Doppler hypothesis of delay means 32 and 34, respectively. The value of m may be any integer greater than one. Further the amount of delay by corresponding delay circuitry, such as 30a, 32a and 34a is generally equal. Delay means 30, 32 and 34 compensate for the differences in correlation time of the correlation signal available from each of correlation means 10, which correlation times are dependent on the frequency of the modulating waveform and the Doppler frequency of the received waveform.
The values of the delay time for each delay circuitry of delay means 30, for example, is selected by the system designer based on criteria such as the Doppler frequency shift expected to be experienced in an actual operating environment and the desired resolution of the system. This may lead to a plurality of delays and corresponding delay circuitry 30.sub.a -30.sub.m (wherein m is an integer greater than one) for Doppler frequency shifts corresponding to relative closing velocities of, for example, from 0 to 20 knots in two knot increments, for a total of ten different delay circuitry 30.sub.a a-30.sub.j for each modulating wave. If it were desired to increase the resolution to one knot increments, then the number of delay circuitry 30.sub.a -30.sub.m could be doubled to twenty for each modulating wave. Because of the generally large spread of possible closing velocities and typically fine velocity resolution desired, this technique may ultimately need to employ a substantial number of delay circuitry 30.sub.a -30.sub.m. Delay circuitry 32.sub.a -32.sub.m, and 34.sub.a -34.sub.m may be configured similarly.
Summer 40, which may include circuitry for adding electronic signals, adds the delayed correlation signals received from each of delay means 30, 32 and 34 for forming an output signal available at the output of summer 40. The output signal supplied by the output of summer 40 includes information in the form of signal amplitude indicative of the degree of correlation, wherein maximum amplitude represents maximum correlation, the relative time of which is indicative of the range to the object. The output signal from summer 40 may be supplied to circuitry (not shown) for additional processing as is known in the art.
Other modulation techniques, such as linear frequency modulation, wherein the modulating wave monotonically changes frequency linearly during the sub-interval, exhibit similar problems when attempting to extract range information from the reflected wave that includes a Doppler frequency shift.
Although closing ranges and approaching objects have been discussed, the invention is not limited thereto but applies equally to increasing ranges and receding objects, though generally these are of less interest.
It would be desirable to provide a system for recovering range information from a plurality of modulated sub-intervals of a pulse of energy wherein the sub-intervals may be contiguous, or from a plurality of modulated spaced apart pulses of energy, wherein, regardless of the form of the modulated energy, the amount of hardware necessary for such recovery could be reduced over that required in prior systems. Further, it would be desirable to provide a method for modulating such pulses and sub-intervals of a pulse of energy such that the results of correlation of each of the reflected modulation waveforms for each of the pulses or sub-intervals may be readily combined for recovering information, such as range to the object, independent of any relative radial velocity between the object and a detector of the reflected energy.
Accordingly, an object of the present invention is to provide apparatus and method for recovering information from a modulated pulse of energy, wherein the amount of hardware for such recovery is reduced over that required in prior systems.
Another object of the present invention is to provide a method for modulating contiguous sub-intervals of a pulse of energy and/or spaced apart pulses of energy such that the results of recovery of the received modulation for each of the sub-intervals and spaced apart pulses from reflected energy, such as from an object, may be readily combined to determine information, such as range, about the object independent of any relative radial velocity between the object and a detector of the reflected energy.